Re: [PATCH v3] libnvdimm, region: sysfs trigger for nvdimm_flush()

[Dan Williams]

On Thu, Apr 27, 2017 at 6:45 AM, Jeff Moyer <jmoyer@xxxxxxxxxx> wrote:
> Dan Williams <dan.j.williams@xxxxxxxxx> writes:
>
>> On Wed, Apr 26, 2017 at 1:38 PM, Jeff Moyer <jmoyer@xxxxxxxxxx> wrote:
>>> Dan Williams <dan.j.williams@xxxxxxxxx> writes:
>>>
>>>> The nvdimm_flush() mechanism helps to reduce the impact of an ADR
>>>> (asynchronous-dimm-refresh) failure. The ADR mechanism handles flushing
>>>> platform WPQ (write-pending-queue) buffers when power is removed. The
>>>> nvdimm_flush() mechanism performs that same function on-demand.
>>>>
>>>> When a pmem namespace is associated with a block device, an
>>>> nvdimm_flush() is triggered with every block-layer REQ_FUA, or REQ_FLUSH
>>>> request. These requests are typically associated with filesystem
>>>> metadata updates. However, when a namespace is in device-dax mode,
>>>> userspace (think database metadata) needs another path to perform the
>>>> same flushing. In other words this is not required to make data
>>>> persistent, but in the case of metadata it allows for a smaller failure
>>>> domain in the unlikely event of an ADR failure.
>>>>
>>>> The new 'flush' attribute is visible when the individual DIMMs backing a
>>>> given interleave-set are described by platform firmware. In ACPI terms
>>>> this is "NVDIMM Region Mapping Structures" and associated "Flush Hint
>>>> Address Structures". Reads return "1" if the region supports triggering
>>>> WPQ flushes on all DIMMs. Reads return "0" the flush operation is a
>>>> platform nop, and in that case the attribute is read-only.
>>>
>>> I can make peace with exposing this to userspace, though I am mostly
>>> against its use.  However, sysfs feels like the wrong interface.
>>> Believe it or not, I'd rather see this implemented as an ioctl.
>>>
>>> This isn't a NACK, it's me giving my opinion.  Do with it what you will.
>>
>> I hate ioctls with a burning passion so I can't get on board with that
>> change, but perhaps the sentiment behind it is that this is too
>> visible and too attractive being called "flush" in sysfs? Would a name
>> more specific to the mechanism make it more palatable? Like
>> "flush_hint_trigger" or "wpq_drain"?
>
> The sentiment is that programs shouldn't have to grovel around in sysfs
> to do stuff related to an open file descriptor or mapping.  I don't take
> issue with the name.  I do worry that something like 'wpq_drain' may be
> too platform specific, though.  The NVM Programming Model specification
> is going to call this "deep flush", so maybe that will give you
> some inspiration if you do want to change the name.

I'll change to "deep_flush", and I quibble that this is related to a
single open file descriptor or mapping. It really is a "region flush"
for giving extra protection for global metadata, but the persistence
of individual fds or mappings is handled by ADR. I think an ioctl
might give the false impression that every time you flush a cacheline
to persistence you need to call the ioctl.